goldenrod (33), grass (34), and leaves as lining (35). 
No pine fasicles or honeysuckle vine were ever 
used in nests in Area I. This corresponds with the 
fact that no pine trees or honeysuckle vine grew in 
this area. Similarly 0.33 of the material in nests in 
Areas I and II consisted of grass whereas this 
formed only 0.12 of the nesting material used in 
Areas III and IV. Again this corresponds to the 
fact that Areas I and II were more open and grassy 
than Areas III and IV. These observations are in 
harmony with other similar ones which support a 
general conclusion that rats utilize for nests only 
such material as is found within the immediate 
vicinity of the nest site. 
A common characteristic of all nests is an 
unorganized base. The materials used in construc- 
tion of the base appear to have been brought in 
and just dumped in one spot. Usually the base is 
composed of coarse stems greater than inch in 
diameter. The simplest type of nest, designated as 
the pad , has only a few flat objects, such as oak 
leaves or bits of paper, laid over the base. Such 
nests were usually just large enough to elevate one 
rat above the floor. Eighty-six (0.473 of the total) 
recorded nests were of this simple type. 
When the initial basal structure is larger, the 
nest gradually assumes a greater degree of organi- 
zation. Frequently, the grass or stems used become 
finer in texture, and are tightly interwoven to 
form a cup-type nest. Leaves or paper is again 
used as a lining. This floor of the nest is elevated 
2 to 6 inches above the floor of the box. Three 
to five rats can sit comfortably in the cavity of 
one of these larger nests. On two occasions seven 
adults were observed in a well-kept nest. Eighty- 
four (.462 of the total) recorded nests were of this 
more complex type. 
Occasionally, the process of weaving is continued 
until it forms a globular hooded nest with a single 
entrance on one side just large enough to provide 
access for one rat. Hooded nests are normally 
associated with the birth of young rats. However, 
by no means do all parturient females achieve this 
most complex level of nest construction. I never 
observed more than three medium-sized adult rats 
in a hooded nest at the same time. Twelve 
(0.066 of total) recorded nests were hooded. 
Organized nests rarely survived simultaneous 
usage by more than five adult rats. Both cup- 
and hooded-type nests become flattened with such 
intensive usage. In contrast to cup- and hooded- 
type nests, which were usually quite clean, the 
flattened nests were fouled with feces. The fact 
that the harborage boxes were so much larger than 
the nest chambers built by the rats in their burrows 
seemed to serve as an additional inducement to- 
ward the formation of harborage aggregates of 
more than seven individuals. These aggregates 
occasionally attained the size of 18 to 20 individuals. 
One of these harborage aggregations, which had 
14 individuals, is shown in figure 57. With these 
larger aggregations, which are usually composed 
of the socially lower ranking individuals, the har- 
borage box soon becomes so filled with nesting 
material that the rats when curled up asleep have 
their backs nearly or actually touching the lid. 
This nesting material becomes a soggy flat mass 
saturated with urine and covered with a thin mat 
of trodden feces until no semblance of a nest 
remains. One possible factor associated with the 
formation of these larger harborage aggregates 
among socially low ranking rats is their smaller 
size. The more rapid heat loss presumably ex- 
perienced by these smaller individuals should be 
ameliorated by their grouping together. In figure 
53 the snow may be seen to have melted off the 
lids of two harborage boxes which contained such 
large aggregations. 
Examples of the several types of nests may be 
seen in figures 52 to 56. 
Beginning shortly after the rats were introduced 
it was always possible to encounter nests in boxes 
where no rats were at the time harboring. Some 
would be in good repair. Those not in good repair 
were frequently found to have been repaired by a 
later date. Usually it was only necessary for the 
rats to add a little lining material to the nest. 
This represents a means by which the rats con- 
ditioned their environment to make it more favor- 
able. When a rat was disturbed by the observer, 
even though this involved only a momentary 
opening of the lid of the box, it would normally 
shift its place of residence. At such times the rat 
could find another nest in a neighboring harborage 
box, which at most needed only slight repair. 
Pad nests inhabited by a single individual were 
occasionally encountered in the patches of honey- 
suckle. Similar nests were encountered at times 
where the traps were piled for storage. An even 
more unusual location for a nest was encountered 
on the campus of the Ohio State University. A 
Norway rat had relined an old robin’s nest some 
12 feet above the ground in a mulberry tree. It is 
the occasional use of such out of the way retreats by 
52 
